'Those queueing up to criticise the Guardian should tell the US to look at its own security, and ask why a junior CIA official turned private contractor possessed so much sensitive information.' Photograph: Uncredited/AP

Britain's spooks are striking back. Weeks, months, after the Guardian, Washington Post and German magazine der Spiegel, published documents about the massive surveillance operations of GCHQ and its close US electronic eavesdropping partner, the National Security Agency, they are saying the leaks have done more damage than the infamous Cambridge spy ring. Indeed, they suggest, the full extent of the damage done may never be fully assessed.

Sir David Omand, a former GCHQ director, told the Times newspaper today that the leaks by former CIA contractor Edward Snowden was "the most catastrophic loss to British intelligence ever, much worse than [Guy] Burgess and [Donald] MacLean in the 50s".

Yet when Vince Cable was asked what he thought about the decision to publish leaked material, the business secretary told BBC Radio 4's Today programme that the Guardian had performed "a very considerable public service". He called for "proper political oversight" of the security and intelligence agencies. Then a few hours later, Nigel Inkster, a former deputy chief of MI6, told BBC Radio 4's World at One programme that the Snowden leaks were "comparable" to those by the Cambridge spies, "only worse". Yet last month he played down the impact of the leaks, described them as "very embarrassing, uncomfortable, and unfortunate". He added, clearly referring to al-Qaida-inspired terrorists: "I sense that those most interested in the activities of the NSA and GCHQ have not been told very much they didn't know already or could have inferred."

It is the job of the spooks to carry out damage assessments after leaks and security and intelligence failures. However, it is unclear whether, as those who make the dramatic comparison with the Cambridge spies suggest, the spooks are concerned more about information getting into the hands of the Russians and Chinese than the "gift" the leaks gave to terrorists, as the head of MI5, Andrew Parker, put it in his speech. Terrorists, of course, pose as much a threat to Russia and China as they do to the UK.

The spooks avoid the issue by describing the damage done by the leaks as "incalculable". It is reminiscent of the Scott "arms-to-Iraq" inquiry which revealed how security and intelligence officials told ministers they had no choice but to sign gagging orders to prevent parliament and MPs know what the spooks were up to. Lord Scott at one point asked a Foreign Office minister about Whitehall's claim that "disclosure of any sources or alleged sources of intelligence information" would cause "unquantifiable damage". Should that be taken as covering "both unquantifiably great and also miniscule". "Yes," replied the minister sternly.

In an exchange about Whitehall's deeply ingrained "need to know" principle, Scott's legal counsel was provoked into telling a Ministry of Defence official: "If you were not told about something, you don't know if you need to know it." Or as John Major explained to the Scott inquiry, what information was given to ministers was always "a value judgment".

That is what ministers are now worried about, and probably prompted Cable's call for political oversight of the spooks. GCHQ, based in Cheltenham, has been left to its own devices, with ministers having little idea what it has been up to. The recent suggestion in the Guardian by former Lib Dem minister Chris Huhne that members of the cabinet and the National Security Council were not aware of the GCHQ-NSA surveillance operations is confirmed by reliable political and official sources who are in a position to know.

There is a lack of effective political, as well as parliamentary and judicial oversight, of the security and intelligence agencies. Meanwhile, instead of attacking the recipients of the leaks, GCHQ, David Cameron, Jack Straw and others now queuing up to criticise the Guardian should tell the US to look at its own security, and ask why a junior CIA official turned private contractor possessed so much sensitive information – collected by British as well as American intelligence agencies.

And it is perhaps ironic, given the dramatic comparison made by Omand and others, that the real damage done by the Cambridge spy ring, as opposed to the embarrassment it caused British ministers and British spooks, has almost certainly been grossly exaggerated.